1. Field of the Invention
The invention relates to a thermal recording element for recording information by thermally deforming the recording layer using high intensity radiation and, more particularly, it relates to a high density thermal recording element having a novel protective layer.
2. Description of the Prior Art
Various recording elements such as silver halide light-sensitive materials and others have been used as recording elements for recording using high intensity radiation such as a laser, etc., such as a recording element wherein the recording layer has a high optical density, which when heated locally in the irradiated portions by absorbing high intensity radiation undergoes physical deformation such as fusion, evaporation or aggregation, whereby information is recorded by the difference in optical density between the irradiated portions and non-irradiated portions. Such a thermal recording element has the advantages that processings such as development, fixing, etc., are unnecessary to form the recorded images, a dark room is unnecessary for recording since the recording layer is not sensitive to ordinary room light, high contrast images are obtained, and further addition recording (add on) of information is possible.
Recording using such a thermal recording element is generally performed by converting the information to be recorded into an electric time-succession signal and scanning the recording element by a laser beam the intensity of which is modulated according to the electric signal. The advantage of this recording method is that the recorded image is obtained in real time.
Recording elements such as described above are reported in, for example, M. L. Levene et al., Electron, Ion and Laser Beam Technology (the records of the 11th Symposium held in 1969), Electronics, p. 50 (March 18, 1968), D. Maydan, The Bell System Technical Journal, Vol. 50, 1761 (1971), C. O. Carlson, Science, Vol. 154, 1550 (1966), etc. Metals, dyes, plastics, etc., are used as the recording layers for the thermal recording elements. In the thermal recording elements using metals as the recording layers, there are elements constructed of a thin layer of a metal such as Bi, Sn, In, etc., on a support which is able to record images with high resolving power and high contrast. However, the recording element having a thin metal layer generally reflects more than 50% of the laser light used for recording making it impossible to utilize the energy of the laser light effectively and hence has such a demerit that the power of the laser light used for recording must be higher, which means the laser source must have a high output for recording at high scanning speed, and thus the recording apparatus used becomes larger and expensive.
Various recording elements having high recording sensitivity have been studied and as an example a recording element comprising a thin layer of Se and Bi and a thin layer of Ge formed thereon for reducing the light reflectance is disclosed in Japanese Patent Publication No. 40479/71. However, the use of Se, etc., is undesirable since there is a possibility of toxicity problems and the recorded images are unsatisfactory.
As another example of a recording element having a reflection preventing layer, the formation of a reflection preventing layer absorbing the light in the wavelength region of the laser on a metal recording layer is disclosed in Japanese Patent Application (OPI) Nos. 151151/75 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") and 74632/76. However, even if a reflection preventing layer is used, it is very difficult to completely eliminate light reflection and even if light reflection could be completely eliminated, a laser light source of high power would be required to produce thermal deformation and hence a recording element having higher sensitivity is needed.
Since the recording layer of the above-mentioned thermal recording element, in particular a metal layer used as the recording layer is liable to scratch, a protective layer is sometimes formed on the recording layer to prevent or reduce scratching. The protective layer must transmit the light beam of high energy density used for recording, possess high mechanical strength, be reluctant to react with the recording layer, exhibit good coating property and be easily formed. As materials used for protective layers, there are inorganic materials such as Al.sub.2 O.sub.3, SiO.sub.2, SiO, MgO, ZnO, MgF.sub.2, CuF.sub.2, etc., and the organic materials disclosed in Japanese Patent Application (OPI) Nos. 96716/74, 59626/76, 75523/76, 88024/76 and 134633/76. However, in recording elements having protective layers possessing effective strength as described in the above specifications the recording sensitivity is greatly reduced in comparison to recording elements having no protective layer. For example, when an effective polymer is used for the protective layer, the thickness of the protective layer must be at least 3 .mu.m in order to function adequately and in this case the recording energy required for recording on the recording element is two to three times the amount of energy required without the protective layer. On the other hand, when the thickness of the protective layer is less than 1 .mu.m, the reduction in recording sensitivity as described above may be avoided but the mechanical strength of the recording element reduces greatly and, in fact, the recording element is not practical.
When an organic high molecular weight material or a polymer is used as the protective layer, it is necessary to prevent the reduction in recording sensitivity as much as possible and at the same time maintain the strength of the protective layer for practical use. When a polymer layer having good adhesive property with the recording layer, having good film-forming property and high softening point, the reduction in sensitivity is great. On the other hand, the formation of a protective layer having high faculty of protecting a recording layer and giving less reduction in recording sensitivity is in balancing properties which conflict with each other.